This invention relates to a programmable logic control system (hereinafter referred to as a "PC"), and more particularly to a programmable logic control system in which a control command from a common control input/output unit is supplied to controllers incorporated in the programmable logic control system and having operation units and input/output sections, thereby controlling a plurality of objects connected to the input/output sections.
In general, programmable logic control systems PC each comprise central processing units (hereinafter referred to as "CPUs") and a plurality of input/output sections (hereinafter referred to as "I/O sections"). Programmable I/O sections, for example, which contain microprocessors and are highly functional, are more and more increasing.
Such highly functional I/O units are used to interface each peripheral unit, i.e. each control input/output unit, with at least one object via the CPU of the control input/output unit. In the case of I/O units which require special programming, they are generally provided with terminals to be connected to a special control input/output unit.
The conventional PC is generally connected to at least one object via the CPU of each control input/output unit. Therefore, the larger the number of highly functional I/O units and hence the greater the amount of information and the contents of processing, the greater the load on the CPU.
Moreover, where a plurality of I/O units which can be directly connected to an exclusive control input/output unit are used, it is also necessary to, for example, change the connection between the control input/output unit and the I/O units.
The CPUs of the conventional PC concentrically manage the interfacing function to interface control input/output units with objects. Since the amount of information to be controlled by the CPUs is more and more increasing these days, the load on the CPUs is accordingly increasing.
On the other hand, in the case of personal computers used as the control input/output units, a multi-task operation in which a plurality of processing or images are simultaneously treated has come to be widely performed in accordance with enhancement of hardware performance.
In light of the above, development of a programmable logic control system is now being desired, which enables control input/output units to execute a plurality of functions, without degrading the performance of the programmable logic control system, in order to perform high-speed data communication.